In recent years, information throughput requirements demanded in a digital electronic device grow steadily. Accordingly, an increase in the signal speed in the digital electronic device is required. Further, capacitance requirements demanded in a printed circuit board are increased and, accordingly, the number of layers required in the printed circuit board and thickness thereof tend to increase. Along with this tendency, the stub length (T-branch portion) is increased in a through hole formed in the printed circuit board for implementation of components or establishment of electrical connection among the front and rear surfaces of the printed circuit board and respective layers. As a result, there arises a problem that signal quality is degraded due to reflection of high frequency components occurring at the portion at which the stub length is increased. To solve this problem, there has been conventionally provided a technique (called “back drilling”) of cutting (removing) the T-branch portion of the through hole using a drill as a printed circuit board fabrication method.
An example of the conventional back drilling technique will be described using FIGS. 59 and 60, which illustrate a cross-sectional view of the inside of a through hole before and after application of back drilling, respectively.
As illustrated in FIG. 59 which illustrates a state before application of back drilling, a through hole 46 is formed by providing a plating layer 48 made of a conductive material in a hole penetrating through a base (base material of a printed circuit board) 47. A lead wire 49 for establishing electrical connection with mounted components and other layers is connected to the through hole 46, and the plating layer 48 perpendicularly extending from the lead wire 49 along the surface of the through hole 46 forms a T-branch 50.
Next, a procedure of a conventional T-branch cutting method will be described. A not illustrated drill is inserted into the through hole 46 from above and cuts the T-branch 50 illustrated in FIG. 59 in the manner as illustrated in FIG. 60. Thereafter, the drill is pulled out of the through hole 46, whereby the cutting of the T-branch 50 is ended.
Thus, by cutting the unnecessary T-branch portion according to the conventional T-branch cutting method, degradation of signal quality due to reflection of high frequency component can be reduced.
Adoption of a build-up board can be taken up as another approach for eliminating the T-branch. However, this approach involves an increase of cost and therefore is not widely used now.
As prior arts relating to the present invention, a printed circuit board fabrication method that cuts a stub (T-branch) without leaving the plating layer of the through hole (refer to, e.g., Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-26549); a working method that forms a groove on the inner surface of a cylinder having a small diameter (refer to, e.g., Patent Document 2: Japanese Laid-Open Patent Publication No. 6-315784); and a laser beam machining apparatus capable of machining an inner face of a tube having a small inside diameter (refer to, e.g., Patent Document 3: Japanese Laid-Open Patent Publication No. 2005-313191) are known.
However, in the conventional back drilling technique, the entire plating layer 48 except for the portion at which the lead wire 49 is provided is removed as illustrated in FIG. 60, so that connectors may not be mounted on both sides of the resultant printed circuit board, which is disadvantageous in terms of mounting efficiency. Further, in the case where the conventional back drilling technique is used, the inner wall surface of the through hole 46 is subjected to cutting at many portions to expose the base 47, so that the amount of moisture absorption through the cut portions of the inner surface of the through hole 46 is increased, which may result in deterioration of substrate characteristics.